feat: address PR review comments — unassign/sync bank questions, blank quiz creation, deactivate redirect, remove duplicate tab titles

- Use Symfony ObjectMapper for BankQuestion/BankAnswer → Question/Answer copy (#[Map(if: false)] on id, season, etc.)
- Track created Question on BankQuestionUsage (nullable FK, onDelete: SET NULL) for unassign/sync support
- Add unassign route: removes the Question copy + usage record
- Add sync route: pushes bank question edits to a finalized-not-started quiz copy
- Auto-sync non-finalized quiz copies on bank question edit; flash warning for finalized-not-started
- Add blank quiz creation (no XLSX required) with new route + template
- Deactivate quiz button now stays on the quiz overview page (redirect_quiz hidden field)
- Remove duplicate h4 titles below the tab bar on all season tabs
- Add migration for bank_question_usage.question_id
- Add Dutch translations for all new strings

templates/backoffice/quiz_add_blank.html.twig

@@ -0,0 +1,28 @@
+{% extends 'backoffice/base.html.twig' %}
+
+{% block breadcrumbs %}
+    <nav aria-label="breadcrumb" class="mb-3">
+        <ol class="breadcrumb">
+            <li class="breadcrumb-item"><a href="{{ path('tvdt_backoffice_index') }}">{{ 'Home'|trans }}</a></li>
+            <li class="breadcrumb-item"><a href="{{ path('tvdt_backoffice_season', {seasonCode: season.seasonCode}) }}">{{ season.name }}</a></li>
+            <li class="breadcrumb-item active" aria-current="page">{{ 'Add blank quiz'|trans }}</li>
+        </ol>
+    </nav>
+{% endblock %}
+
+{% block body %}
+    <div class="row">
+        <div class="col-md-6 col-12">
+            <h2 class="mb-3">{{ t('Add a quiz to {name}', {name: season.name})|trans }}</h2>
+            {{ form_start(form) }}
+            {{ form_row(form.name) }}
+            {{ form_widget(form.save, {attr: {class: 'btn btn-primary'}}) }}
+            {{ form_end(form) }}
+        </div>
+        <div class="col-md-6 col-12">
+            <p class="mb-3">{{ 'Create an empty quiz and add questions from the question bank.'|trans }}</p>
+        </div>
+    </div>
+{% endblock %}
+
+{% block title %}{{ parent() }}Backoffice{% endblock %}